1. Field of the Invention
The present invention relates to a spring for supporting a support frame of a shadow mask in a color cathode ray tube and more particularly, to a spring for a color cathode ray tube that is capable of compensating for the displacement of a shadow mask according to a thermal expansion of the support frame for the shadow mask, thereby preventing mislanding of electron beams that may be caused from the displacement of the shadow mask.
2. Description of the Related Art
With the development of the technology for a cathode ray tube, recently, a large-sized color cathode ray tube becomes popular, which causes weight of a support frame for a shadow mask to be increased. However, this causes an amount of shock resistance, upon dropping, to be undesirably increased, such that an excessive shock may be applied to a spring that is adapted to connect a stud pin of a panel with the shadow mask and to the shadow mask. This results in the deviation of the support frame for the shadow mask and the deformation of the shadow mask. In addition, the shadow mask suffers a thermal expansion, which evokes the change in the landing of the electron beams. This results in the degradation of the color purity.
Therefore, there is a need for an effective buffering against the shock applied upon a dropping test and the thermal expansion of the shadow mask, for the purpose of reducing the amount of the shock resistance and preventing the degradation of the color purity.
FIG. 1 is a partly cut sectional view illustrating a general color cathode ray tube.
As shown, the general cathode ray tube includes: a panel 1 formed on the front surface of the cathode ray tube and provided with a luminous fluorescent material 2 of colors R, G and B on the inside thereof; a funnel 4 fused on the rear end of the panel 1; a shadow mask 6 on which beam transmitting holes of a dot or slot shape are formed for separating the colors; a support frame 8 for supporting the shadow mask 6, such that the shadow mask 6 is separated by a predetermined interval from the panel 1; an earth magnetic shield 10 coupled to the support frame 8; a spring 12 coupled to the support frame 8, for buffering the shock applied to the shadow mask 6 due to a thermal expansion of the shadow mask; a stud pin 14 formed on the side wall 1a of the panel 1, for coupling the spring 12 to the panel 1; an electron gun 18 inserted into the neck of the funnel 4, for emitting electron beams 16 to the luminous fluorescent material 2; a deviation yoke 20 for adjusting the advancing orbits of the electron beams 16, such that the electron beams 16 are scanned to the luminous fluorescent material 2; and a reinforcing band 22 for preventing the width contraction caused from external shocks.
The spring 12, as shown in FIGS. 4a to 4c is comprised of: a stud pin coupling part 12a on which a hole xe2x80x98hxe2x80x99 adapted to be inserted and fixed into a stud pin 14 on the side wall 1a of the panel 1 is formed; a frame welding part 12b for coupling on the support frame 8; and an inclined part 12c coupled to the stud pin coupling part 12a and the frame welding part 12b, while being inclined at a predetermined angle and having folded faces 12d and 12e inclined at a predetermined angle (xcex8) on the coupled parts with the stud pin coupling part 12a and the frame welding part 12b. 
Under the above construction, the electron beams of the colors R, G and B emitted from the electron gun 8 inserted into the neck of the funnel 4 are adjusted in the orbits thereof by means of the deflection yoke 20 and then passed through the beam transmitting holes of the shadow mask 6. Next, the electron beams are scanned horizontally and vertically in a sequential order to the luminous fluorescent material 2 of the colors R, G and B spread on the inside of the panel 1 to thereby emit the light from the luminous fluorescent material 2. As a result, image is formed and displayed.
At this time, only about 30% of the electron beams 16 emitted from the electron gun 18 are passed through the beam transmitting holes of the shadow mask 6 and those of the remainder 70% are scanned to the part where no hole is formed on the shadow mask 6.
A part of the electron beams 16 scanned on the shadow mask 6 is reflected. In this case, a part of the electron beams reflected is changed to shock energy and then absorbed on the shadow mask 6. The absorbed energy enables the movement of the molecules in the interior of the shadow mask 6 to be activated, which induces the friction among the molecules. The friction generates heat from the shadow mask 6.
As a consequence, the temperature of the shadow mask 6 rises and the volume thereof becomes expanded according to a thermal expansion coefficient of the material of the shadow mask 6. The result is shown in FIG. 2. As shown, the shadow mask 6 is expanded toward the luminous fluorescent material 2 relative to the direction of a tube shaft.
In the figure, a reference number xe2x80x986xe2x80x99 represents a position where the shadow mask 6 before the thermal expansion is fixed and xe2x80x986axe2x80x99 represents a position where the shadow mask 6 after the thermal expansion is moved.
Under the above state, the heat generated from the shadow mask 6 is conducted to the support frame 8, whereby the temperature of the support frame 8 rises. This enables the support frame 8 to be thermally expanded. Hence, the support frame 8 is expanded toward the side wall 1a of the panel 1 and the shadow mask 6 expanded is moved to the direction opposite to the luminous fluorescent material 2 relative to the direction of the tube shaft.
If the volume of the support frame 8 is larger than that of the shadow mask 6, however, the shadow mask 6 is excessively moved to the direction opposite to the luminous fluorescent material 2 relative to the direction of the tube shaft, which results in the mislanding of the shadow mask 6. In this case, an amount of variation in the landing of the shadow mask 6 is xcex94LE, as shown in FIG. 3.
In order to decrease the amount of variation in the landing xcex94LE, the support frame 8 has to be made of a material having a low thermal expansion coefficient. However, this causes the cost of the product to be undesirably high. As an alternative, hence, the spring 12 is designed in many ways in the structure thereof, such that the position of the shadow mask 6 is corrected to decrease the amount of variation in the landing xcex94LE.
FIG. 3 is an enlarged view of the part xe2x80x98axe2x80x99 of FIG. 1. As shown, the hole, which is formed on the end of the one side of the spring 12, is inserted and coupled to the stud pin 14 formed on the center of the side wall 1a of the panel 1, and the end of the other side of the spring 12 is welded and coupled to the rectangular type of support frame 8. And, a skirt (which is not shown in the drawing) having an end portion folded along the periphery of the hole formation part (on which the beam transmitting holes of a dot or slot shape are formed, which is not shown in the drawing) of the shadow mask 6 is welded and fixed on the inner wall of the support frame 8.
In FIG. 3, a reference numeral 16a represents the electron beams moved by a predetermined interval from the orbits of the electron beams 16 and 8a represents the support frame 8 moved by a predetermined interval from the position thereof before the thermal expansion.
On the other hand, the spring 12 having the inclined folded faces is varied according to the thermal expansion of the support frame 8, such that the stage difference between the stud pin coupling part 12a, the frame welding part 12b and the inclined part 12c is generated.
FIG. 4a shows the spring 12 having the inclined folded faces at the state where the stud pin coupling part 12a, the frame welding part 12b and the inclined part 12c are disposed in parallel with each other in a length direction of the spring 12, that is, at the state where the stage difference between the stud pin coupling part 12a, the frame welding part 12b and the inclined part 12c in a width direction of the spring 12 is zero. Reference numerals 12d and 12e represent the folded parts where the folding is formed according to the change of the shape of spring 12, 12f represents the hole that is coupled to the stud pin 14, and 12g represents welding points to which the spring 12 is welded and fixed on the support frame 8.
FIG. 4b shows the spring 12 at the state where the frame welding part 12b and the inclined part 12c are folded perpendicularly (xcex8=90xc2x0) along the folded part 12d and the inclined part 12c and the stud pin coupling part 12a are folded perpendicularly (xcex8=90xc2x0) along the folded part 12e in the direction opposite to the folded part 12d, thereby forming a maximum stage difference therebetween.
FIG. 4c shows the spring 12 having the inclined folded faces at the state where the stage difference value corresponding to the intermediate value between the values in FIGS. 4a and 4b is obtained.
According to the spring that has the inclination angle at which the stage difference is generated according to the thermal expansion, an amount of variation of the frame welding part 12b relative to the center of the hole 12f of the stud pin coupling part 12a is given by the following equation (1):
xcex94Y=(Lxc3x97sin xcex8)/2 
wherein, the xe2x80x98Lxe2x80x99 denotes the length of the inclined part 12c and the xe2x80x98xcex8xe2x80x99 denotes the inclination angle of the inclined part 12c. 
In FIG. 4c, the xe2x80x98xcex94Yxe2x80x99 represents an amount of displacement of the frame welding part 12b relative to the center of the hole 12f of the stud pin coupling part 12a at the time when the stage difference corresponding to the intermediate value is generated.
FIG. 5 is an exemplary view illustrating the variation in the landing of the electron beams relative to the displacement of the conventional shadow mask. A symbol xe2x80x98B1xe2x80x99 denotes an initial position of the shadow mask at an initial state, xe2x80x98B2xe2x80x99 denotes the position of the shadow mask after the thermal expansion, xe2x80x98B3xe2x80x99 denotes the position of the shadow mask after the correction action of the spring. Reference numerals 16a, 16b and 16c represent the electron beams, symbols LP1, LP2 and LP3 represent the landing points of the electron beams, and symbols S1, S2 and S3 represent the positions where the electron beams are passed through the beam transmitting holes of the shadow mask.
Before the shadow mask 6 is thermally expanded, it is disposed on the position B1 and the electron beams 16a are passed through the position S1 to scan the luminous fluorescent material 2. At this time, the landing point of the electron beams 16a is the LP1.
Under the above state, when the support frame 8 for the shadow mask 6 is expanded, the shadow mask 6 is moved to the direction opposite to the luminous fluorescent material 2 relative to the tube shaft and thus disposed on the position B2. And, the electron beams 16b are passed through the position S2 to scan the luminous fluorescent material 2. At this time, the landing point of the electron beams 16b is the LP2.
At this state, if the position of the support frame 8 for the shadow mask is corrected by the elastic operation of the spring 12 having the inclined folded faces, the shadow mask 6 is moved toward the luminous fluorescent material 2 and thus placed on the position B3. Thus, the electron beams 16c are passed through the position S3 to scan the luminous fluorescent material 2. At this time, the landing point of the electron beams 16c is the LP3.
The position of the shadow mask 6 is corrected by an amount of correction of the position xcex94C, with a consequence that the mislanding corresponding to the difference of the landing points (LP1xe2x88x92LP2) is corrected by the difference of the landing points (LP3xe2x88x92LP2).
The amount of correction of the position xcex94C is determined upon parameters such as the folded angle of the spring 12 having the inclined folded faces, the number of folded faces, the thermal expansion coefficient of the support frame 8, the material of the shadow mask 6. Thus, the value of the amount of correction of the position is not constant.
Therefore, the dimension of the spring in the conventional cathode ray tube is determined in consideration of the above parameters, but it is difficult to obtain the spring having an optimum dimension due to the strength of the spring and the work process thereof, which causes the mislanding of the electron beams due to the thermal expansion of the support frame for the shadow mask.
Hence, the spring in the conventional cathode ray tube forms inclined faces folded at a predetermined angle at the coupling parts thereof, such that it can absorb a part of the shock according to the increment of the weight of the shadow mask, thereby achieving a good buffering effect against an external shock. However, the degradation of the color purity caused due to the variation of the landing of the electron beams according to the thermal expansion of the support frame for the shadow mask can""t be corrected.
Accordingly, it is an object of the present invention to provide a color cathode ray tube having a spring structure capable of compensating for the displacement of a shadow mask frame, thereby suppressing the degradation of the color purity according to the mislanding of electron beams.
To accomplish this and other objects of the present invention, there is provided a color cathode ray tube that comprises a spring formed by bonding a plurality of metal members having different thermal expansion coefficients in a length direction relative to a tube shaft.